We talked recently on the phone about hum in my zen kit1.I am sending you pictures of my amp. Please take a look if you have a chance.I have tried different things like changing source, changing interconnects, changing power outlets(use the same outlet/ use different outlets), changing positions of units and cables, but result stay the same - after attaching interconnects to any source hum starts and hum sound stays the same whatever I do (no hum at all without source). I am under impression that something happened inside my amp after a few month of work. I don’t remember this hum in the beginning.Waiting for your advice, what should I do.

Your board is grounded in two locations which could be causing your problem. You have the chassis grounded to the board with the brown wire, but then you also have a wire from the left input jack ground connecting to the chassis. Remove this wire.

Also, the wires connecting each input jack to the board should ideally be shielded with the shields connected at the input jack ground on the circuit board.

I think the key is "after connecting interconnects" the hum starts. That is what happens with me. My amp has a single earth. A grounded shield on the input cables did not help me.

I think this means you have a ground-loop, or mains contamination of the source signal. Having done some reading diagnosing these is trickier than it sounds.

Build a ground-lifter (a plug/socket with the earth disconnected) to use simply to diagnose the problem by breaking loops. A ground-lifter may not be 100% safe as a final solution. And try powering various components in the hifi chain unearthed to mains (as long as one component is earthed there will be a ground connection for all components through the shield of the RCAs.)

This did not work for me.

My next step is to get an electrician to check out the Ze of our household earthing and the earth-bonding (expensive equipment and know-how required). My understanding is that it is not good simply to connect your own earthiing rod, but that Incomer Earth as it is called should be formally tested and fixed if it is inadequate. Have any building contractors been working near your house recently?

To exclude a ground loop, lift the ground on your cd player (use a socket/plug with the earth disconnected) and attach it to your amp with RCA cable. Leave your amp earthed. Disconnect everything else from your hifi chain.

Switch both on. If the hum is still there and unchanged then it is NOT a ground loop because you have broken the signal return path through earth which might have been causing the loop.

I have tried this but for me the hum is still there. I believe this means either there is earth contamination from some other electrical device, or earth is inadequate. I have tried a few possible culprits for contamination without success (central heating boiler etc).

It seems there is no simple test for adequacy of your earth - this is a job for an electrician. I'm going to speak to the electricians at work to see what they suggest.

None of mine either, sll: the hum doesn't show up in my T-Amps, my Push-pull valve amp, nor my solid state amp. However apparently SET topology is extremely sensitive to mains / earth borne interference. I think this is partly to do with the air-gapped output transformers, push-pull valve amps do not require these, and so are much less susceptible to hum.

One of the reasons that SETs tend to be more expensive than other amp types is the design, and component costs required to suppress this interference (and also to ensure good frequency extension).

I have been "tweaking" my Zkit1 for a while now, gradually coaxing more and more power out of it. Somewhere along the way, the hum got so loud that I couldn't ignore it anymore. Today I looked into it, and found a fix that virtually eliminated the hum I was experiencing on the bench. Unfortunately there is still audible hum when the amp is in system, but it is substantially lower than what it use to be.

Two 14 gauge solid wires (out of a chunk of Romax) connected to the RCA input connector shields on one end and ground on the other - where the ground connects to the negative terminals of the two (3 in my case) large power supply capacitors (which is where I ground the volume controls). The solid wire worked quite a bit better than any stranded wire I tried, and the ground point by the power supply capacitors worked much better than going straight to the earth ground connection.

I don't think this was a ground loop problem. The hum was dependent on the volume control setting (more volume, more hum) and went away when the RCA input cable was unplugged. On the bench, when I had my signal generator connected as the source, the hum was present when the signal generator was connected to the Zkit1 even when the signal generator was turned off and unplugged. After installing the two solid wires there was no hum when the signal generator was connected.

Dank - sounds like a ground loop to me - when you unplug the RCA you break the loop (at the RCA shield) and the hum goes away.

Try just connecting the LIVE centre conductor of the RCA. If you still hear sound then you have a return path through the chassis earth rather than the RCA shield, and will therefore have a loop when the RCA is fully connected.

Whether that loop is then the source of the hum is another question.

It is possible to buy high-end RCA cables which are designed with the shield disconnected at one end. This forces the return path to earth, and will break a loop.

I do have a loop - but breaking the loop does not change the hum in any way.